With the market expansion of mobile electronic devices such as cellular phones, laptop personal computers and digital cameras, a long-life secondary battery having high energy density is growingly developed as a cordless power source of these electronic devices.
An electrode active material of structural elements of the secondary battery is a substance directly contributing to electrode reactions of a charge reaction and a discharge reaction in the battery, and has a central role in the secondary battery. That is, the electrode reaction in the battery is a reaction which occurs associated with giving and receiving of electrons by applying a voltage to an electrode active material electrically connected to an electrode located in an electrolyte, and the electrode reaction in the battery progresses during charge and discharge of the battery. Accordingly, as described above, the electrode active material systemically has a central role in the secondary battery.
Thus, in recent years, organic materials having an oxidation-reduction activity receive attention as this kind of a material for an electrode active material. In the organic materials, it is thought that since multi-electrons of two or more electrons can be involved in the oxidation-reduction reaction, by using such a characteristic for an electrode reaction in the battery, a secondary battery having a larger capacity density than inorganic materials can be achieved.
For example, the document 1 proposes an electrode for a battery which has a structural unit represented by the following formula (1′):—(NH—CS—CS—NH)  (1′)
and includes a rubeanic acid or rubeanic acid polymer capable of being coupled with lithium ions.
The rubeanic acid or rubeanic acid polymer containing a dithione structure represented by the general formula (1′) is coupled with lithium ions during reduction, and releases the coupled lithium ions during oxidation. It is possible to perform charge and discharge by using such a reversible oxidation-reduction reaction of rubeanic acid or rubeanic acid polymer. In the document 1, for example, a secondary battery having a capacity density of about 400 Ah/kg was obtained in the case of using rubeanic acid for the positive electrode active material.
On the other hand, the electrode active material of the secondary battery varies significantly in volume according to chemical changes associated with the charge-discharge reaction, and consequently, the electrode active material in a solid state may be destroyed or dissolved in an electrolyte solution so that it does not function as the electrode active material. That is, in the secondary battery using an organic material in the electrode active material, although the electrolyte solution is prepared by dissolving an electrolyte salt such as a Li salt in a solvent, charging and discharging is performed by using an oxidation-reduction reaction of a molecule itself, and therefore the electrode active material is easily dissolved in the electrolyte solution in contrast to a lithium ion secondary battery performing charging and discharging in a state of maintaining a crystal system. Thus, the suppression of such dissolution of the electrode active material in the electrolyte solution is investigated.
For example, the document 2 proposes a positive electrode including a mixture, wherein the mixture contains an active sulfur and an electron conductor which is mixed with the active sulfur and is configured so that electrons can migrate between the active sulfur and the electron conductor, an ion conductor which is mixed with the active sulfur and is configured so that ions can migrate between the active sulfur and the ion conductor and in the mixture, the utilization of the active sulfur for an electrochemical reaction is approx. 10% to approx. 100%.
In the document 2, the positive electrode active material is formed of a mixture of a sulfur element, a polymer electrolyte such as a polyethylene oxide having ion conductivity and an electron conductive substance such as polyaniline, and an aprotic liquid such as sulfolane, dimethyl sulfone, dialkyl carbonate, tetrahydrofuran (THF), dioxolan, propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), butyrolactone, N-methylpyrrolidinone, tetramethylurea, glyme, ether, crown ether and dimethoxyethane, is used for a solvent of the electrolyte solution, and thereby, a battery cell is formed.
Further, the document 3 proposes a battery including a negative electrode, a solid composite positive electrode containing an electrically active sulfur-containing substance, and an electrolyte interposed therebetween.
The document 3 describes, as a preferred embodiment of an electrolyte, a mixture of one or more ionic electrolyte salt and one or more electrolyte solvent selected from the group consisting of N-methylacetamide, acetonitrile, carbonates, sulfolane, sulfone, N-alkylpyrrolidone, dioxolan, aliphatic ethers, cyclic ethers, glyme and siloxane. Further, in the document 3, 1,3-dioxolan is used as the electrolyte solvent and dimethoxy ethane is used as the ionic electrolyte salt to prepare an electrolyte, and a battery in which a positive electrode material contains a substance containing electrically active sulfur, is prepared.
The document 1: JP No. 2008-147015A (claim 1, par. [0011], FIG. 3, and FIG. 5)
The document 2: JP No. 09-511615A (claim 1, pp. 32-33)
The document 3: JP No. 2002-532854A (claim 1, claim 83, pars. [0031] and [0088])
In the document 1, although the two-electron reaction is initiated by using the rubeanic acid having a dithione structure, when a low-molecular-weight compound such as a rubeanic acid is used, dissolution in an electrolyte solution or contamination of the electrode due to a dissolved compound easily occurs, as described above, and therefore the battery lacks the stability for repeated charge and discharge. Further, when a polymer compound like a rubeanic acid polymer is used, interactions between molecules within the rubeanic acid polymer are large, while dissolution in an electrolyte solution or contamination of the electrode can be suppressed. Therefore, the movement of ions is interfered with and the proportion of the active material to be used effectively is reduced.
In the document 2 and the document 3, although a sulfur-based compound is used for the positive electrode active material and an electrolyte solution having sulfolane, dioxolan or the like used as the solvent is prepared, and thereby a battery is formed, it is in a difficult situation to attain a secondary battery having stable and excellent cycle characteristics even though using such an electrolyte solution.
Although as described above, a secondary battery is prepared by combined use of an organic compound and an electrolyte respectively indicated in the prior art, it is not yet possible to achieve a long-life secondary battery which has adequately high energy density and high output, and has excellent cycle characteristics.
The present invention has been made in view of such a situation, and it is an object of the present invention to provide a secondary battery having a high capacity density and high output, and excellent cycle characteristics with small deterioration of capacity even in repeating charge and discharge, and a method for charging and discharging a secondary battery.